Metal cans are broadly divided into two-piece and three-piece cans. A two-piece can is made of two sections, namely a can end and a can body integrated with a can bottom. A three-piece can is made of three sections, namely a can body, a can end, and a can bottom. A two-piece can body has no seam (weld) and thus looks good, but is usually required to endure a high strain level. A three-piece can has seams in its can body and thus is inferior to a two-piece can in the appearance, but is usually not required to endure a high strain level. Therefore, in the market, two-piece cans are often used for containing low volumes of quality products, and three-piece cans are often used for high volumes of low-priced products.
Of two-piece cans, those for containing aerosols are highly strained by drawing, and elongated in the height direction (hereinafter may be referred simply as “have a high strain level”). Two-piece can bodies are usually made of a thick and expensive aluminum sheet, and are rarely made of a thin and low-cost tin or tin-free steel sheet. The reason for this is that a steel sheet is not suitable for making a two-piece can body for containing an aerosol which is produced through drawing or DI processing at high strain levels, while a soft metal material such as an aluminum endures impact forming.
Under the above-described circumstances, production of a two-piece can body with a high strain level from a thin, low-cost, and high-strength tin or tin-free steel sheet will be industrially meaningful.
In the prior art, common two-piece cans with a low strain level are produced from a resin-laminated steel sheet (hereinafter referred to simply as “laminated steel sheet”) through drawing or DI processing. Laminated steel sheets used to make two-piece cans with a low strain level are commonly coated with a polyester. Typical examples of the coating include ionomer compounds composed mainly of polyethylene terephthalate, polyethylene terephthalate-isophthalate copolymer, polyethylene terephthalate-polybutylene terephthalate copolymer, or saturated polyester.
These laminated steel sheets are designed according to the methods for making two-piece cans with a low strain level, and are suitable for such applications. However, there has been no study on the use of the laminated steel sheet for the production of a can body produced through drawing and subsequent diametral reduction with a high strain level, such as a two-piece can for containing an aerosol.
Patent Documents 1 to 3 disclose a method for drawing a resin-coated metal sheet and a method for DI processing. However, all the techniques are aimed at can bodies with a low strain level, such as those for containing beverages and foods. Specifically, these documents disclose a technique for relaxing the internal stress, which has been caused by processing during the manufacture of a two-piece can with a low strain level, through heat treatment after the processing, and a technique for promoting the orientation of the resin. Patent Documents 2 and 3 disclose performance of heat treatment at the intermediate or final step thereby preventing the delamination of the resin layer and improving the barrier properties after the processing. More specifically, Patent Document 2 suggests heat treatment of an orientational thermoplastic resin for the relaxation of internal stress and the promotion of orientational crystallization. The procedure of the heat treatment is commonly used for producing beverage cans. According to Patent Document 2, the heat treatment is given to a cup after redrawing, and is preferably carried out at a temperature 5° C. lower than the melting point of the coating resin thereby thoroughly promoting the crystallization. However, the examples of the invention imply that the invention is applicable only to can bodies with a low strain level. The examples of Patent Document 3 disclose a method including forming a coating layer composed of a saturated polyester and an ionomer compound, followed by DI processing. Patent Document 3 provides a method including heat treatment after drawing, followed by DI processing, necking, and flanging. However, as is the case with Patent Document 2, the examples of the invention imply that the invention is applicable only to can bodies with a low strain level.
Patent Documents 4 and 5 describe methods including heat treatment of a formed can at a temperature not lower than the melting point of the resin thereby relaxing the internal stress. However, according to the main text and examples of the description, the strain level of the can body thus obtained is still low.
Patent Document 1: Japanese Examined Patent Application Publication No. 7-106394
Patent Document 2: Japanese Patent No. 2526725
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-148324
Patent Document 4: Japanese Examined Patent Application Publication No. 59-35344
Patent Document 5: Japanese Examined Patent Application Publication No. 61-22626
More specifically there has been no disclosed method for producing a two-piece can body with a high strain level, such as those for containing an aerosol, from a laminated steel sheet. The inventors had attempted to make a two-piece can body with a high strain level through the steps of forming a circular laminated steel sheet into a bottomed tube by DI processing, and then reducing the diameter of the vicinity of the opening. As a result of this, the resin layer caused delamination and fracture. Since these problems are incident to forming at a high strain level, the inventors thought that heat treatment was effective for solving these problems. However, it is known that these problems cannot be sufficiently solved by heat treatment before or after the forming, and delamination of the resin layer cannot be avoided at such a high strain level. Therefore, the delamination of the resin layer during the production of a two-piece can body with a high strain level cannot be prevented by the simple use of the prior art. In addition, the processability of the resin layer deteriorated during or after the heat treatment.